Lyme disease (LD) is a tick-borne, multi-system, infectious disorder caused by the extracellular spirochetal bacterium Borrelia burgdorferi (Bb). Since Bb lacks exotoxins or known bacterial secretory system, it is widely believed that the inflammatory manifestations of the disease result from the host's innate and co-evolving adaptive immune responses to the bacterium. Monocytes and macrophages are considered to be critical cellular elements of the innate immune response to the spirochete. For more than a decade, Bb-mediated cell activation was thought to occur chiefly as a result of the interactions of the spirochete's abundant outer membrane-associated lipoproteins with CD14 and Toll-like receptors (TLR) 1/2 on the surface of these cells. We now have extensive evidence that phagocytosis of live spirochetes by human monocytes and murine macrophages generates a more intense and far broader inflammatory response than can be attributed to lipoprotein-mediated, cell surface TLR1/2 activation. Phagocytosis of intact Bb also induced transcription of interferon-2 (IFN-2) and type I interferon-stimulated genes (ISGs), independently of TLR2. Recent evidence from the investigator's laboratory provides substantial evidence the TLR2-independent signaling events elicited by Bb in human monocytes are MyD88-dependent and occur via TLR8. On the basis of our collective findings we now propose a new model of Bb-induced monocyte activation, which emphasizes the importance of phagocytosis and the cooperative role of TLR2 and TLR8 signaling. In this model, binding of Bb to the monocyte/macrophage cell surface, through a yet to be characterized phagocytic receptor, is followed by a broad sequence of immune signaling events which mechanistically can only be integrated following internalization of the bacterium and formation of the phagolysosome. The validation of the phagosomal signaling model, the centerpiece of our research strategy, will significantly enhance our understanding for how the bacterium triggers the inflammatory processes that under actual disease conditions cause tissue damage and/or that promote bacterial clearance. To accomplish our goals and examine mechanistic aspects of the proposed model, we have formulated the following Specific aims. In Aim 1 we will use a ex vivo stimulation technique to characterize in detail the mechanisms by which phagocytosis of Bb elicits activation and TLR1/2 and TLR8 dependent inflammatory signals in human monocytes. In Aim 2, we will examine key elements of the phagosomal signaling model in Bb-infected human and murine macrophages. The use a large repertoire of available knockout mice will allow us to define mechanistically how Bb is sensed and triggers immune responses in these cells. In Aim 3 we will characterize MyD88 dependent and independent responses to Borrelia burgdorferi both in vitro and in vivo using human monocytes and macrophages obtained from children and adults with known congenital deficiencies in components of the MyD88 signaling pathway (IRAK-4 and MyD88 deficient).